1. Field of the Invention
This invention relates to Linear Deflection Amplifiers for use in conjunction with cathode ray tube (CRT) displays for radar systems. More particularly, this invention relates to a linear deflection amplifier characterized by transadmittance, transimpedance feedback, and resonant flyback energy recovery for and controlling the magnetic deflection yoke used in a radar CRT display terminal.
2. Description of Related Art
The output display of many radar systems is a cathode ray tube (CRT) upon which information is displayed and measured. Various modes of displays are used. One form of presentation is a type-P display characterized by a PPI (Plan Position Indicator) which shows range and azimuth for a full 120.degree.. The center of the CRT screen represents the location of the radar antenna. Radar information is commonly stored in a memory whose address represents range and azimuth (angle). The memory is loaded radially (constant azimuth) and sent out along an arc (constant range). This forms on the display a PPI (plan position indicator) display. A typical display is 120.degree. forming a fan shaped display with raster lines along the arc.
PPI and other modes of radar displays are characterized by horizontal and vertical deflection circuits which control a CRT electron beam as it sweeps across the screen in accordance with a predetermined arc or raster scan pattern. The electron beam is deflected by a magnetic field created by currents passing through horizontal and vertical deflection yokes.
Retrace signals are produced by an external timing circuit.
The deflection circuits include two power amplifiers for driving two yokes, one for vertical and the other for horizontal deflection. Prior art deflection magnetic yoke amplifiers were characterized by an amplifier in a feedback circuit which saturated when a step input of sufficient magnitude was applied to the amplifier input, breaking the feedback loop. Small amplitude signals do not disturb the closed loop system. It was the closed loop amplifier response that established the visual display of the CRT, as well as linearity and general fidelity of the image.
Generally two types of inputs to the sweep circuits are most commonly encountered. These inputs are the small signal step input and the ramp. The small step input may be used to move the indicator beam small distances, while the ramp input is used to scan the beam across the CRT screen. One requirement for horizontal sweep amplification circuitry has been the need to provide retrace of the deflected signal, so that the indicator beam may return to its point of origin and begin the next scan across the screen. In the prior art, about a five micro-second settling time was imposed upon the deflection amplifier. Such predetermined settling response time allowed relatively slow amplifier systems to handle a five microsecond retrace when magnetic deflection was used. This is the case of a majority of television and radar monitors. This retrace capability was known as resonant flyback.
In the prior art, the linear amplifiers which amplified the horizontal sweep signal used the resonant frequency of the deflection yoke, a capacitor and a switch to achieve retrace and generate a fast reverse voltage when hit by a ramp voltage return. Resonant flyback caused the yoke to ring for one half cycle of resonant frequency and was independent of the amplifier bandwidth.
In the prior art, the amplifier was not linear during flyback, as it was during forward deflection. During flyback, the prior art amplifier designs were not suitable for beam positioning. The amplifiers were only useful to return the beam to the starting point during horizontal flyback time.
Prior art linear amplifiers for use in conjunction with a magnetic deflection yoke of a CRT system have been complex circuits and generally provided controlled deflection current by using bipolar transistors. These bipolar transistors were usually configured in an emitter-coupled complementary configuration (PNP and NPN). Cross-over distortion was often an undesirable but necessary result of this complementary circuit configuration. Cross-over distortion was high for small input signals. The relative distortion diminished as the input signal became larger. However, for very large signals, saturation of the complementary configured bipolar transistors occurred and distortion again rose sharply due to clipping of the signal. Cross-over distortion is common in bipolar complementary or class B stage configurations of linear amplifiers because the basic operation of such amplifiers requires one transistor to be turned "on" and operating during one half cycle, while the other transistor is turned "on" during the other half cycle. As the input sinusoidal or other waveform crosses the horizontal axis, distortion occurs because current flows in both transistors for a short period. The residual biasing of the class B configuration of the bipolar transistors leads to this distortion.
Furthermore, temperature compensation is needed in high power circuits where current is shuttled between complementary branches of an amplifier configuration. Heretofore, these problems of cross-over distortion and temperature compensation have not been directly addressed in deflection amplifier systems used to drive magnetic deflection yokes of a radar CRT system.